Traditional axial exhaust used on through-air dryers and bonders (i.e., prior art shelled roll dryers and bonders), such as those used in the paper manufacturing industry, has flow limitations based on the available open area in the end plane of the dryer cylinder head as controlled by the head open area to meet structural demands. The traditional axial exhaust also has limitations with regard to non-uniform air flow, high energy usage and space impact. These limitations can impinge on the uniformity of drying, result in higher energy costs and impact space usage in a facility.
During a drying or thermal bonding process, heated air (heated process air) passes through a wet web traveling on the rotating cylinder (the dryer cylinder or foraminous shelled cylinder). The heated process air travels through the web and between the shell openings. Typically, moisture in the web or the web itself cools the heated air so that the temperature inside the shell is cooler than that of the heated air applied to the web. This cooler but still hot air travels radially through the foraminous shell and then axially through the exhaust. During the drying process, the heated air typically ranges from about 120 to about 290 degrees Celsius and cools to about 80 to about 260 degrees Celsius after passing through the traveling web and picking up moisture.
The exhaust, referred to herein as the “exhaust head” with respect to prior art devices, in a prior art through-air dryer or bonder typically has a tapered shape, the taper becoming narrower the further from the dryer end plane. See, for example, part no. 210 of FIG. 1A of U.S. Pat. No. 8,656,605 to Parker for an illustration of the prior art exhaust head. This shape results in increasing air (gas) velocities as the exhaust narrows to the axial exit. This variation in air flow velocity is problematic at high air flows as it results in non-uniform air flow across the web with non-uniform velocities found at the edges of the web being dried or bonded. This results in non-uniformity in product drying and properties. Further, this limitation often forces the dryer design to be a double end exhaust to allow for sufficient airflow volume to meet the production requirements. A double ended exhaust requires having exhaust air exiting on the tend side (i.e., the operator side) of the dryer. This hinders fabric changes and access to the tend side of the dryer. Even when double end axial exhaust is used, the limiting factor for increased air flow above normal limits is the head open area. Thus, inherent to the design of the prior art exhaust heads is the resulting high velocities. The high velocities create pressure losses. High energy usage is required to overcome this pressure loss and total air flow volume.
The exhaust head design in prior art dryers also creates limitations with regard to space requirements by, for example, limiting options with regard to placement of exhaust ducting. Versatility with regard to placement of exhaust ducting will allow for a greater range of install options for paper manufacturers.
In prior art dryers, the head could be redesigned to achieve lower exhaust duct (head radial) velocities by, however, it's impractical, costly and time consuming to design new heads for each design scenario. Further, a linear (same diameter as dryer roll) exhaust would adversely affect drying at the edge of the web by bleeding drying gas from the edge of the web.
A solution that allows greater air flow capacity with greater uniformity of airflow while taking into account space limitations inherent in the prior art design will result in greater uniformity in drying while increasing the production capacity of the dryer or thermal bonder with lower energy costs and better space utilization over prior art designs.